Zhabs kyu is the second of the four Tibetan vowel signs and writes the vowel u. Alone among the four it is written beneath the consonant rather than above it, and its name records the position: zhabs is the honorific word for foot, and the sign hangs below the letter it belongs to.
Like the other three it replaces the inherent a, so ka with zhabs kyu is read ku, and like them it cannot stand by itself. Where a syllable begins with u the sign is written under a chen, the thirtieth consonant. Shown alone it is displayed on a dotted circle, the convention for a dependent mark.
Its position under the letter puts it in company with the subscripts, and a syllable can carry both: a root letter with a subjoined ya or ra and a zhabs kyu beneath the whole. The stack is assembled in a fixed order and read as one unit, and the vowel sign is the last element to attach.
The honorific in the name is worth noticing in passing. Tibetan maintains a separate honorific vocabulary, and the grammatical tradition drew on it in naming the parts of its own writing, which is a small indication of the standing writing was given.
Because it is the only one of the four written below the letter, zhabs kyu is also the only one that competes for space with a subscript, and a stack carrying a subjoined consonant and a zhabs kyu together is correspondingly deep. In a printed line that depth is one of the things that gives Tibetan its distinctive texture on the page.